Electron acceptor bioremediation

Where the terminal electron acceptor is not present initially in sufficient quantity, addition of oxygen for aerobic bioremediation can be as simple as bubbling air into the aquifer addition of electron acceptors for anaerobic bioremediation is more complex and can foster concerns regarding the toxicity and fate of the added material. 

However, the many pathways by which MTBE and other oxygenates may be biodegraded anaerobically have been the subject of recent research and ongoing studies. Table 24.12 highlights the various electron acceptors that are used in anaerobic bioremediation studies and contrasts the products of complete anaerobic degradation with those for aerobic metabolism. 

Bioremediation usually requires a procedure for stimulation of and maintaining the activity of microorganisms. For biodegradation to be successful, it is necessary to provide a continuous supply of a suitable electron acceptor (such as oxygen or nitrate), nutrients (nitrogen, phosphorus), and a carbon source for energy and cell material. The most commonly deficient components in the subsurface are eiectron acceptors and nutrients. 

The location, distribution, and disposition of chemical contaminants in the aquifer can strongly influence the likelihood of success for bioremediation. This technology generally works well for dissolved contaminants and contaminants adsorbed onto higher-permeability sediments. However, if the majority of the contamination is trapped in lower-permeability sediments or outside the flow path, where it is in contact with nutrients and electrons acceptors, this technology will have reduced impact, or none at all. 

Once a bioremediation effort is started, the bioreactions that occur in the presence of added electron acceptors will result in significant variations of water chemistry across the three-dimensional area of the aquifer. Careful monitoring of these variations is an important indicator of the effectiveness of the remediation process. 

In its basic form, bioremediation of the vadose zone involves introduction of nutrients and electron acceptors necessary to stimulate the indigenous bacteria and provide for removal of waste products generated by the reactions. This sometimes takes the form of a series of injections of a soup of nutrients and electron acceptors into the vadose zone through wells, or infiltration galleries. Other sites may require pressure fracturing of the soil before the stimulant blend can be injected. 

During the past several years, the number of successful bioremediation projects has steadily increased. Remediation professionals have experimented with many techniques, some more effective than others. The following paragraphs discuss some of the more efficient procedures used to introduce nutrients and oxygen (or other electron acceptors). 

Without appropriate cleanup measures, BTEX often persist in subsurface environments, endangering groundwater resources and public health. Bioremediation, in conjunction with free product recovery, is one of the most cost-effective approaches to clean up BTEX-contaminated sites [326]. However, while all BTEX compounds are biodegradable, there are several factors that can limit the success of BTEX bioremediation, such as pollutant concentration, active biomass concentration, temperature, pH, presence of other substrates or toxicants, availability of nutrients and electron acceptors, mass transfer limitations, and microbial adaptation. These factors have been recognized in various attempts to optimize clean-up operations. Yet, limited attention has been given to the exploitation of favorable substrate interactions to enhance in situ BTEX biodegradation. 

BTEX bioremediation projects often focus on overcoming limitations to natural degradative processes associated with the insufficient supply of inorganic nutrients and electron acceptors. However, other limitations associated with the presence and expression of appropriate microbial catabolic capacities may also hinder the effectiveness of bioremediation. Thus, while subsurface addition of oxygen or nitrate has proven sufficient to remove BTEX below detection levels [134,145,292,315,316], it has been only marginally effective at some sites [6]. Sometimes, the concentration of a target BTEX compound fails to decrease below a threshold level even after years of continuous addition of nutrients and electron acceptors [317]. This phenomenon has also been observed for many other xenobiotic and natural substrates under various experimental conditions [327-332]. 

System failure for in situ bioremediation efforts is often the result of ineffective transport of nutrients and electron acceptors due to channeling into preferential flow paths, heterogeneities, adsorption, biological utilization, and/or chemical reactions in the soil. Many of these problems can be overcome using electric fields for transport and injection instead of conventional groundwater injection by hydraulic techniques. 

Figure 3. Common bioventing system for treatment of vadose-zone contaminants using oxygen as a terminal electron acceptor. Reprinted from In Situ Bioremediation When Does It Work Copyright 1993 by the National Academy of Sciences. Courtesy of the National Academy Press, Washington, DC.

Anaerobic processes are now known to be much more diverse in biodegradation of pollutants than was thought even a few years ago. Anaerobic bioremediation of nitro-substituted compounds, halogenated molecules, and even hydrocarbons now appears possible, employing electron acceptors such as nitrate, halogenated... 

Reinhard, M. (1993). In situ bioremediation technologies for petroleum- derived hydrocarbons based on alternate electron acceptors (other than molecular oxygen). In In Situ Bioremediation of Ground Water and Geological Material A Review of Technologies, ed. R. D. Norris et al., section 7, pp. 7.1-7.7. EPA/600/R-93/124. NTIS Document No. PB93-215564, Washington, DC. 

Groundwater and Soil. Pumping out the liquid phase is an obvious first step if die contaminant is likely to be mobile, but in situ bioremediation is a promising option. Thus, the U.S. Department of Energy is investigating the use of anaerobic in situ degradation of carbon tetrachloride with nitrate as electron acceptor, and acetate as electron donor. 

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